Interpretive Summary: Toxoplasmosis is major food safety and public health problem. The disease is caused by the single celled parasite, Toxoplasma gondii. Although the parasite affects virtually all warm-blooded animals anf humans, its life cycle is completed only in cats. Any member of the cat family can shed environmentally resistant stage of the parasite, the oocyst. Cats become infected by preying on infected rodents and birds. The parasite multiplies extensively in the intestine of cats, eventually producing oocysts. Nothing is known of the events that control the growth of the parasite in the cat gut (enteric stages). In the present paper we characterized genes associated with enteric stages of Toxoplasma. The results will be of interests to biologists and parasitologists and hopefully can lead to interventions that can curtail or stop parasite multiplication in cat intestine.

Technical Abstract:
Considerable work has been carried out to understand the biology of tachyzoites and bradyzoites of Toxoplasma gondii, which infect a range of intermediate hosts, in large part due to in vitro culture methods for these stages. However, culturing methods for stages in the definitive felid host, including the merozoite and sexual stages, have not been developed hindering the ability to study a large portion of the parasite’s life cycle. Here, we begin to unravel the molecular aspects of the merozoite stage by providing comprehensive data on gene expression. To profile gene expression differences in enteric stages, we harvested merozoites from the intestine of infect cats and hybridized mRNA to the Affymetrix Toxoplasma GeneChip. We analyzed the merozoite data in context of the life cycle by comparing it to previously published data for the oocyst, tachyzoite, and bradyzoite stages (Fritz HM et al. 2012). Principal component analysis highlighted the unique profile ofthe merozoite samples, placing them approximately half-way on a continuum between the tachyzoite/bradyzoite and oocyst samples. Prior studies have shown that antibodies to surface antigen one (SAG1), also known as p30, and many dense granule proteins do not label merozoites: our microarray data confirms that these genes were not expressed at this stage. Also, the expression for many rhoptry and microneme proteins was drastically reduced while the expression for many surface antigens was increased at the merozoite stage. Gene Ontology and KEGG analysis revealed that genes involved in transcription/translation and many metabolic pathways were upregulated at the merozoite stage, highlighting unique growth requirements of this stage. To functionally test these predictions, we demonstrate that an upstream promoter region of a merozoite specific gene is sufficient to control expression in merozoites in vivo. Merozoites are the first developmental stage in the coccidian cycle that takes place within the gut of the definitive host. The data presented here describe the global gene expression profile of merozoite stage and the creation of transgenic parasite strains that show stage-specific expression of reporter genes in the cat intestine. These data and reagents will useful in unlocking how the parasite senses and responds to the felid gut environment to initiate coccidian development.